1. Field of the Invention
The present invention relates in general to an electron microscope, and in particular concerns an improvement of an electron microscope in which contrast of microscopic images is improved with various aberrations being concurrently reduced.
2. Description of the Prior Art
In the electron microscopes, and among all in a conventional transmission electron microscope, the image contrast is attained by limiting the divergence angle of electron beams scattered by the specimen. It will be helpful for having a better understanding of the present invention to describe at first a hitherto known approach in some detail. Referring to FIG. 1 of the accompanying drawings which shows schematically and partially a lens system of a hitherto known electron microscope, there is inserted a movable objective aperture member 3 on an optical axis O in a gap between magnetic pole pieces of a conventional objective lens 2 on the side of an imaging lens 4 as viewed from the side of a specimen 1. The objective aperture member 3 serves for limiting a divergence angle .alpha. of electron beam 6 scattered by the specimen 1 which beam will hereinafter be referred to as the scattered electron beam. To this end, the objective aperture member 3 is usually formed of a metallic thin membrane and has a circular bore 7 formed therein with a diameter on the order of 20 .mu.m. Implementation of the circular bore 7 of a diameter smaller than 20 .mu.m will encounter a great difficulty in fabrication and thus is impractical. Further, the objective aperture member 3 having such an extremely fine bore is likely to incur degradation of the image quality due to contamination of the aperture member or the like factors, rendering the use of such aperture member impractical.
However, in a certain type of the electron microscope, and particularly in a high resolution electron microscope in which an objective lens of a short focal length is employed, there exists a demand for further decreasing the divergence angle .alpha. of the scattered electron beam 6 by resorting to the use of the objective aperture member 3 having the bore 7 of a smaller diameter. Nevertheless, it has been practically impossible to realize an optimal diameter for the objective aperture member in view of the difficulty in attaining the desired dimensional accuracy as well as the possibility of involving degradation in the image quality.
On the other hand, in the application in which the electron microscope is operated at a small magnification on the order of several thousand or less magnifications, there is known a method of improving the contrast of image according to which the divergence angle .alpha. of the scattered electron beam 6 is restricted by displacing a back focal plane of the objective lens 2 to a plane of an aperture for restricting field of view (which will hereinafter be referred to as the field restricting selected area aperture and is shown in FIG. 3) by varying correspondingly the magnetic excitation of the objective lens 2. This method is however disadvantageous in that the spherical aberration coefficient Cs as well as the chromatic aberration coefficient Cc tends to be significantly increased due to the fact that the magnetic excitation of the objective lens 2 is changed to an extremely low excitation level, resulting in that the quality of image is remarkably deteriorated to a great disadvantage, when observation is to be conducted at several thousand magnifications.
Moreover, because the objective aperture member 3 is usually inserted between the pole pieces of the objective lens, the inter-pole gap (i.e. the gap between the pole pieces of the objective lens) can not be decreased in order to assure a desired angular range for permitting a tilt or inclination of the specimen, which in turn gives rise to a troublesome problem that the spherical aberration coefficient Cs and the chromatic aberration coefficient Cc become very difficult to decrease, in addition to a problem that the vacuum state in the vicinity of the specimen is likely to be deteriorated.